Priming of the decomposition of ageing soil organic matter: concentration dependence and microbial control

The amount of carbon (C) stored in soil is an important regulator for the global climate and soil fertility and is the balance between formation and decomposition of soil organic matter (SOM). Decomposition of SOM can be powerfully affected by labile carbon (C) supplements in, for example, the rhizosphere. A stimulation of SOM mineralisation induced by labile C additions is termed priming', and the mechanisms for this phenomenon remain elusive. The most widely held explanation assigns priming to successional dynamics in r- and K-selected groups within the microbial community; groups which have also been connected with fungal (K-selected) and bacterial (r-selected) decomposers. New evidence has also suggested that recently formed SOM is particularly sensitive to priming. We investigated (i) the labile C concentration dependence of SOM mineralisation, (ii) the susceptibility of differently aged SOM to priming and (iii) if priming is due to bacterial or fungal growth dynamics. To create an age gradient of traceable SOM, we spiked a pasture soil using C-14 glucose, and subsampled plots 1day, 2months, 5months and 13months after application (i.e. SOM aged 1day - 13months). Glucose (0-4000g C g(-1)) was added in subsequent laboratory experiments, and respiration, SOM mineralisation ((CO2)-C-14 evolution), bacterial growth rates (leucine incorporation) and fungal biomass (ergosterol) were tracked during ca. 1week. Mineralisation of SOM aged 2-13months showed similar labile C concentration dependencies, and priming increased mineralisation of SOM systematically by up to 350%. The glucose treatments induced variable microbial growth responses for differently aged SOM, which were unrelated to the priming effect. That successional dynamics in microbial r- and K-selected groups, or bacterial and fungal decomposers, respectively, underpinned priming was incompatible with the results obtained. An alternative explanation could be that SOM transformation by extracellular enzymes, for subsequent respiration, could be triggered by labile C. In conclusion, labile C primed the mineralisation of 2-13months aged SOM, and the mechanism for this priming was unrelated to microbial growth dynamics.